Hybrid powertrain with geared starter motor and belt alternator starter and method of restarting an engine

ABSTRACT

A hybrid powertrain has an engine, a starter motor, and a gear train that connects the starter motor with the engine, and a motor/generator. A belt drive train connects the motor/generator with the engine. The powertrain has a first energy storage device with a first operating range of voltage and a second energy storage device with a second operating range of voltage at least partially in common with the first operating range of voltage. A controller places a switching device in an on-state so that the first energy storage device is connected with the second energy storage device or in an off-state in which the first energy storage device is disconnected from the second energy storage device. The controller causes the switching device to be in the off-state and the starter motor and the motor/generator to be powered with energy from the first energy storage device to restart the engine.

TECHNICAL FIELD

The invention relates to a hybrid powertrain that has both a startermotor and a motor/generator used for an engine restart.

BACKGROUND

Hybrid electric vehicles offer reduced fuel consumption and emissions byutilizing both an electric motor/generator and an internal combustionengine and an onboard controller programmed to vary use of each of theengine and motor/generator during different driving conditions toachieve peak efficiency.

One type of hybrid electric vehicle is referred to as abelt-alternator-starter hybrid vehicle. This type of vehicle has amotor/generator operatively connected to an engine crankshaft by a beltand pulley system. The motor/generator is used to start the engine froma key start and may be recharged by the engine during regenerativebraking “Key start” or “first start” refers to the engine being startedfor the first time after the engine has been completely shut off for anextended time. A key start is typically initiated by a vehicle operatorinserting a key into the ignition and turning the key to a key-up orfurther to a key-crank position. This is in contrast to the “quickstart” or “restart” which occurs after the engine has been temporarilyshutdown by the electronic controller as driving conditions allow, suchas when the vehicle is stopped at a stop light. This helps to reduceemissions and increase fuel economy. A high voltage battery (e.g., a 120volt battery) is typically used to provide sufficient power toaccomplish a restart and is also used during regenerative braking.

Most hybrid vehicles also employ a low voltage battery (e.g., a 12 voltbattery) to run typical motor vehicle accessories, such as headlights,an audio system, the ECU and other electronic components. The lowvoltage battery may also be recharged by the high voltage battery (viaan auxiliary power module (“APM”), also referred to as a DCDC converter,that converts power from the high to the low voltage) assuming the APMhas been activated (which is only after the engine is running). As usedherein, “activating” the APM means causing the APM to begin convertingvoltage from the high voltage level to a lower voltage level, or viceversa.

Different hybrid motor vehicles may utilize the high and low voltagebattery systems differently in performing a key start of the hybridmotor vehicle. In one configuration of a hybrid motor vehicle, the highvoltage battery is used to perform the key start and the low voltagebattery is retained for powering vehicle accessories but is not utilizedin the key start process. In another configuration, the low voltagebattery is utilized in a key start by supplying power to a starter motorto start the engine, with the high voltage battery utilized for restartsbut not for key starts. The low voltage battery also powers the motorvehicle accessories after the key start. In yet another hybrid vehicleconfiguration, the starter motor initially turns the engine to a firstspeed during the key start and the motor/generator is powered by thehigh voltage battery and is then employed in tandem with the startermotor to assist in turning the engine to a final start speed. In someconfigurations, the APM converts energy stored in the high voltagebattery to the voltage level of the low voltage battery when the lowvoltage battery is used in a key start.

SUMMARY

A powertrain is provided with two energy storage devices selectivelyconnectable by a switching device to control power flow to a startermotor and a motor/generator during engine restarts and other operatingmodes, such as key starts and for charging of the energy storagedevices. Both of the energy storage devices are used under mostoperating conditions. This allows the powertrain to function with arelatively small motor/generator and with energy storage devices oflower voltage than typically used on hybrid vehicles, reducing theoverall cost of the powertrain without reducing drive quality and whileretaining fuel economy benefits.

Specifically, a hybrid powertrain is provided that includes an engine, astarter motor and a motor/generator. A gear train operatively connectsthe starter motor with the engine. A belt drive train operativelyconnects the motor/generator with the engine. The powertrain has a firstenergy storage device and a second energy storage device. The firstenergy storage device is operable within a first range of operatingvoltage having a first minimum voltage and a first maximum voltage. Thesecond energy storage device is operable within a second range ofoperating voltage having a second minimum voltage and a second maximumvoltage. The first range of operating voltage is at least partially incommon with the second range of operating voltage in that the secondmaximum voltage is greater than the first minimum voltage and less thanthe first maximum voltage.

A switching device is selectively operable in an on-state and anoff-state. The first energy storage device and the second energy storagedevice are operatively connected with one another when the switchingdevice is in the on-state and are operatively disconnected from oneanother when the switching device is in the off-state. A controller isoperatively connected to the switching device, the starter motor and themotor/generator. The controller is operable to cause the switchingdevice to be in the off-state and to power both the starter motor andthe motor/generator with the first energy storage device during arestart of the engine. The starter motor and the motor/generator areoperatively connected with only the first energy storage device duringrestarting of the engine when the switching device is in the off-state.The switching device may be in the on-state when an operating parameterof the powertrain is different than a predetermined reference parameterby at least a predetermined amount. For example, if the engine operatingtemperature or operating temperature of one of the energy storagedevices is below a predetermined temperature, the switching device maybe in the on-state. This may allow the second energy storage device,which may be a lead-acid battery, to be used during relatively coldtemperature restarts.

In one embodiment, the controller is operable to cause both the startermotor and the motor/generator to be used in tandem throughout the entireengine restart. In another embodiment, only the starter motor is poweredat the beginning of the restart, and then the motor/generator is poweredduring the restart event only after a predetermined amount of time haspassed since initiation of the restart or after the engine has reached apredetermined speed. The controller may also determine when the restartis complete, and cause the switching device to be in the on-state if itis not already in the on-state.

The controller may also be operable to advantageously coordinateoperation of the switching device, the motor/generator and the startermotor to manage power flow in other vehicle operating states, such asduring key starts, for charging of one or both energy storage devices,for regenerative braking, and for preparing for a subsequent restartfollowing an automatic shutdown of the engine.

The above features and advantages and other features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a hybrid powertrain with a startermotor and a motor/generator operatively connected to an engine; and

FIG. 2 is a flow diagram of a method of restarting the engine of FIG. 1.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numbers refer to likecomponents throughout the several views, FIG. 1 shows a portion of avehicle 10 with a hybrid powertrain 11 that includes an internalcombustion engine 12 with an output member 14, such as a crankshaft,operatively connected to an input member 16 of a transmission 18. Thetransmission 18 includes a gearing arrangement and clutches (not shown)through which torque flows from the input member 16 to an output member20 and through a final drive 21 to vehicle wheels 23 to propel thevehicle 10. The wheels 23 shown may be front wheels or rear wheels. Asecond pair of wheels that is not shown may also be powered by thepowertrain 11, or may be unpowered.

The engine 12 includes a crankcase 22 and a cylinder block 24. An engineflywheel 25 rotates with the crankshaft 14. Two different components forstarting the engine 12 are operatively connectable to the crankshaft 14.A starter motor 26 is operatively connectable to the crankshaft 14through a gear train 28. The gear train 28 includes a starter gear 30and an external gear 34 on the flywheel 25. The starter gear 30 isconnected for rotation with a shaft 31 of the starter motor 26. Asolenoid 32 is selectively energizable to move the starter gear 30 alongthe shaft 31 into meshing engagement with the external gear 34 on theflywheel 25.

A motor/generator 36 is also operatively connectable to the crankshaft14 for starting the engine 12. The motor/generator 36 is operativelyconnected to the crankshaft 14 by a drive train 37 that includes a belt38 that engages with a pulley 40 connected to rotate with a shaft of themotor/generator 36 and with a pulley 42 connected to rotate with thecrankshaft 14. The motor/generator 36 may be referred to as abelt-alternator-starter, and may be configured to be powered by 12volts. In another embodiment, the drive train 37 includes a chain inlieu of the belt 38 and sprockets in lieu of the pulleys 40, 42. Bothembodiments of the drive train 37 are referred to herein as a “beltdrive train”. The motor/generator 36 has an integrated power inverter 39that converts direct current provided by one or both energy storagedevices 50, 52 to alternating current needed to power themotor/generator 36 to function as a motor, and converts alternatingcurrent to direct current to be directed to the energy storage devices50, 52 when the motor/generator 36 functions as a generator.

A first energy storage device 50 and a second energy storage device 52are used separately or together under different operating conditions toprovide power for a key start or for a restart of the engine 12. Thefirst energy storage device 50 may be a lithium ion battery. The firstenergy storage device 50 may have a nominal voltage of 3.75 volts percell with four cells for a total nominal voltage of 15 volts, while thesecond energy storage device 52 may have a nominal voltage of 2 voltsper cell with six cells for a total nominal voltage of 12 volts. Thefirst energy storage device 50 is operable in a first range of operatingvoltage having a first minimum voltage and a first maximum voltagebetween its terminals 51, 53. The first minimum voltage of the firstenergy storage device 50 may be 2 volts per cell for a total minimumvoltage of 8 volts, such as during a key start. The first maximumvoltage for the first energy storage device 50 may be 4.25 volts percell for a total maximum voltage of 17 volts. The first maximum voltageof the first energy storage device would only be achieved underregenerative braking discussed herein, using the motor/generator 36,with the switching device 54 in the off-state. When the regenerativebraking event ends, the voltage in the first energy storage device 50would fall to approximately 15 volts and the switching device 54 wouldbe placed in the on-state again.

The second energy storage device 52 may be a lead-acid battery. Thesecond energy storage device 52 may have a nominal voltage of about 12volts with a second range of operating voltage between its terminals 55,57 having a second minimum voltage of about 1.33 volts per cell with sixcells total for a total minimum voltage of about 8 volts during a keystart. The second maximum voltage for the second energy storage device52 may be approximately 2.5 volts per cell for a total second maximumvoltage of 15 volts, such as during rapid charging. Thus, the first andsecond ranges of operating voltage are at least partially in common,i.e., overlap or share at least some voltage range, as the secondmaximum voltage is greater than the first minimum voltage and less thanthe first maximum voltage. If the first energy storage device 50 is alithium-ion battery, its operating life is increased if kept near thelower end of the first range of operating voltage. If the second energystorage device 52 is a lead-acid battery, it will have its longestoperating life if kept near the upper end of the second range ofoperating voltage. Accordingly, the first and second energy storagedevice 50, 52 are designed to function well together in the range ofoverlap of their range of operating voltages.

A switching device 54 is selectively controllable to operate in anoff-state, represented by an open position shown, and an on-state,represented by the closed position 56, indicted in phantom. Theswitching device 54 may be a normally-open (i.e., normally in theoff-state) device, a normally-closed (i.e., normally in the on-state)device, a bi-stable (capable of remaining in either the on-state oroff-state without outside signal) device, an electromechanical device,or a solid state device. In the on-state, the switching device 54completes a circuit to operatively connect the starter motor 26, themotor/generator 36 and the first energy storage device 50 with thesecond energy storage device 52, so that both the starter motor 26 andthe motor/generator 36 are operatively connected with both the firstenergy storage device 50 and the second energy storage device 52. Whenthe switching device 54 is in the off-state, the starter motor 26 andthe motor/generator 36 are operatively connected with the first energystorage device 50 but not with the second energy storage device 52. Avehicle electrical system 58 is continuously operatively connected withthe second energy storage device 52, and is only operatively connectedto the first energy storage device 50 when the switching device 54 is inthe on-state. Accordingly, when the switching device 54 is in theoff-state, such as during regenerative braking or an engine restart, asdiscussed below, the brightness of lights in the vehicle electricalsystem 58 will not be affected and the voltage range that the vehicleelectrical system 58 is exposed to is controlled to a relatively narrowrange.

A controller 60 is operatively connected to the switching device 54, toboth of the energy storage devices 50, 52, to the starter motor 26, tothe solenoid 32, to the engine 12, and to the motor/generator 36, asindicated with dashed lines. The controller 60 includes a processor 62with a stored algorithm that determines how the controller 60 controlsoperation of the starter motor 26, the motor/generator 36, and theswitching device 54 based on vehicle operating conditions measured orestimated from input signals provided by sensors, not shown. The vehicleoperating conditions may include temperature of the engine 12,temperature of the energy storage device 50, temperature of the energystorage device 52, speed and torque at the crankshaft 14, and speed andtorque at the output member 20.

Based on the input signals received by the controller 60, the processor62 determines whether the switching device 54 should be in the on-stateor the off-state, whether the starter motor 26 should be powered,whether the solenoid 32 should be energized, and whether themotor/generator 36 should be energized and operated as a motor or as agenerator. The controller 60 then sends control signals to the variouscomponents to cause them to operate in accordance with the storedalgorithm and the vehicle operating conditions.

For example, if the controller 60 determines that a key start has beeninitiated, the switching device 54 is placed in the on-state and thesolenoid 32 engages the starter gear 30. The starter motor 26 performsthe entire key start (i.e., the entire time that an ignition key is in aclosed position) using energy supplied from both of the energy storagedevices 50, 52. The motor/generator 36 is not powered during the keystart. Immediately following the key start, the engine 12 runs or idles,i.e. does not shut off, until its temperature and the parameters (e.g.temperature and charge) of the energy storage device 50 meetpredetermined parameters. The switching device 54 remains closed. Alsoimmediately following the key start, if the charge of the second energystorage device 52 is below a predetermined level, then themotor/generator 36 is controlled to function as a generator to provideelectricity to gradually charge the second energy storage device to apredetermined level of charge. The switching device 54 remains in theon-state in order for this charging to occur.

When the vehicle 10 is braking, the motor/generator 36 generateselectricity at the maximum rate possible based either on its power limitor based on the torque limits imposed by the belt drive 37, drivequality, or the maximum rate that electricity can be accepted by theenergy storage devices 50, 52 and vehicle electrical system 58 or by thefirst energy storage device 50 alone. During a regenerative brakingevent, the switching device 54 may be in the on-state. If the voltagerises to the maximum for the second energy storage device 52 (e.g. 15volts) or the maximum for the accessories in the vehicle electricalsystem 58 (e.g. 14 volts) and the state of charge of the first energystorage device 50 is below a predetermined level (e.g., less than 75%),then the controller 60 places the switching device 54 in the off-stateand the motor/generator 36 charges only the first energy storage device50 up to its maximum voltage (e.g., 17 volts) or a predetermined maximumstate of charge (e.g., 90%). The controller 60 then reduces or ceasesfunctioning of the motor/generator 36 as a generator so that the amountof electricity produced by the motor/generator 36 remains within thepredefined limits of maximum voltage and maximum state of chargetolerated by the first energy storage device 50.

When the temperature of the engine 12, and the temperature and state ofcharge of the first energy storage device 50 are sufficient, and if theengine 12 is not required to drive the vehicle 10 (such as when thevehicle 10 is near zero speed with the brake pedal depressed) then theengine 12 shuts down (referred to as an automatic shutdown) and thesolenoid 32 engages the starter gear 30. This engagement may take placebefore the crankshaft 14 has stopped rotating to facilitate meshing ofthe gears 30 and 34. When the engine 12 shuts down, the motor/generator36 may have been charging the first energy storage device 50 duringregenerative braking, so that the switching device 54 may be in theoff-state. If so, then the switching device may remain in the off-state,awaiting a subsequent restart of the engine 12, potentially with the twoenergy storage devices 50, 52 at different voltages (e.g. 15 volts forthe first energy storage device 50 and 12 volts for the second energystorage device 52).

When the need for the engine 12 is imminent (such as when the brakepedal is released), the switching device 54 is placed in the off-stateif it is not already in the off-state. The solenoid 32 has previouslybeen energized to hold the starter gear 30 engaged, and the startermotor 26 and the motor/generator 36 are then energized via the firstenergy storage device 50 to rotate the engine 12. The starter motor 26and the motor/generator 36 may each operate as motors during the entirerestart of the engine 12, or the starter motor 26 alone may initiate therestart, and the motor/generator 36 will then be controlled to functionas a motor to assist with the restart only after a predetermined amountof time has passed since the restart was initiated (e.g., since thestarter motor 26 was energized), or only after the speed of the engine12 has reached a predetermined speed. As the engine 12 is beingre-started, the voltage of the first energy storage device 50 will drop,and then the voltage will recover as the load lightens and disappears.At some point, the voltage of the first energy storage device 50 willcome back up to the same voltage or near the same voltage as the secondenergy storage device 52. In one embodiment, the controller 60 placesthe switching device 54 in the on-state at that time. If the voltage ofthe first energy storage device 50 does not come back up near the samevoltage as the second energy storage device 52, then the controller 60may cause the motor/generator 36 to generate electricity to raise thevoltage of the first energy storage device, so that the switching device54 may be placed in the on-state without lowering the voltage of thesecond energy storage device 52 and the vehicle electrical system 58.

Specifically, a method 100 of controlling the powertrain 11 using thecontroller 60 is shown as a flowchart in FIG. 2. The method 100 beginswith block 102, in which vehicle operating conditions are determined.The vehicle operating conditions are as described above, and arecalculated or estimated based on input signals received by thecontroller 60 from sensors and other known components. For example, ifthe vehicle operating conditions determined in block 102 indicate that akey start of the engine 12 has been initiated, such as by a signal thata key has been turned in the vehicle ignition, then the solenoid 32 isenergized to engage the starter gear 30 in block 104 and the switchingdevice 54 is placed in the on-state in block 106. The starter motor 26is energized in block 108 when the key is turned in the ignition tocomplete an electrical circuit with both energy storage devices 50, 52providing power to the starter motor 26. The engine 12 then runs oridles until the engine temperature and the temperature and state ofcharge or level of charge of the first energy storage device 50 reachpredetermined levels as determined in block 110, with themotor/generator 36 functioning as a generator to charge the first energystorage device 50. In block 112, it is determined whether the state ofcharge or level of charge of the second energy storage device 52 is at apredetermined level. If it is not, then the motor/generator 36 iscontrolled to function as a generator to charge the second energystorage 52 device to a predetermined state of charge or charge level inblock 114. The switching device 54 remains in the on-state during thistime. When the second energy storage device 52 reaches a predeterminedstate of charge or level of charge, the method 100 returns to block 102.

If the controller 60 determines in block 102 that the vehicle 10 isdecelerating, it may control the motor/generator 36 to operate as agenerator in a regenerative mode in block 116 to assist in slowing thewheels 23. If the voltage provided by the motor/generator 36 rises tothe maximum level of operating voltage that is acceptable for the secondenergy storage device 52 of the vehicle accessory system 14, then, ifthe switching device 54 is in the on-state, it is placed in theoff-state in block 118. The second energy storage device 52 is thusactively isolated from the motor/generator 36 during regenerativebraking, and the second energy storage device 52 and vehicle electricalsystem 58 are thus not subjected to the high voltage levels that may besuitable for the first energy storage device 50. In block 120, if it isdetermined that the first energy storage device 50 has reached itspredetermined maximum level of operating voltage or predeterminedmaximum state of charge, operation of the motor/generator 36 as agenerator is then reduced (due to voltage) or ceased (due to state ofcharge). The method then returns to block 102.

If it is determined in block 102 that the engine 12 is not required topropel the vehicle 10, and if the temperature of the engine 12 and thestate of charge and level of charge of the first energy storage device50 are at or above predetermined levels, then the engine 12 is shutdownin block 122 and the solenoid 32 is energized to engage the starter gear30 in block 124. At this point, the vehicle 10 may still be deceleratingor may be at a standstill (e.g., stopped at a traffic light). If thecontroller 60 receives an input signal indicating that vehicleacceleration is imminent, such as by the release of the brake pedal,then the switching device 54 is placed in the off-state if it is notalready in the off-state and, in block 126, the starter motor 26 isenergized using power from the first energy storage device 50 to restartthe engine 12. The motor/generator 36 is also energized in block 128. Inone embodiment, block 128 occurs simultaneously with block 126 so thatboth the starter motor 26 and the motor/generator 36 provide torqueduring the entire restart (i.e., from an engine speed of zero to fullidle speed). In another embodiment, block 128 occurs only after thestarter motor 26 has caused the engine 12 to reach a predetermined speedor after a predetermined amount of time has passed since the startermotor 26 was energized. This allows the starter motor 26 to assist withthe restart during the higher torque, lower speed beginning portion ofthe restart and the motor/generator 36 to assist during the lowertorque, higher speed latter portion of the restart. The motor/generator36 may thus be designed to provide only a relatively low torque,enabling a cost savings over motor/generators that provide a highertorque output.

With the switching device 54 in the off-state, only the first energystorage device 50 is available to provide electric power for restartingthe engine 12. The second energy storage device 52 may be a lead-acidbattery, which provides good starting at relatively cold temperatures,but is not required during a restart event at relatively warmtemperatures. Thus, by keeping the switching device 54 in the off-state,the energy storage device 52 is isolated from both the starter motor 26and from the motor/generator 36 and is not used during the restart.Optionally, if the operating temperature of the first energy storagedevice 50 is below a predetermined temperature, then the switchingdevice 54 may be placed in the on-state so that the second energystorage device 52 is also used during the restart, as it may be alead-acid battery better suited for low temperature performance.

Optionally, in block 130, the torque provided by the motor/generator 36during block 128 may be modulated at a compression pulse frequency ofthe torque provided by the engine 12 to smooth the torque at thecrankshaft 14, improving start quality. The compression pulse frequencyat which the motor/generator 36 is modulated may be a predeterminedfrequency or may be based on actual feedback indicative of the operatingcompression pulses. In block 128, when the controller 60 determines thatthe restart is complete, such as by determining that the engine 12 isoperating at a speed at or above idle speed, the method 100 returns toblock 102.

If the controller 60 determines in block 102 that the fuel has beenshutoff to the engine 12 during deceleration, and relatively lightacceleration is subsequently required that does not require a restart ofthe engine 12 and can be accommodated by the motor/generator 36 alone,then in block 132, the controller 60 controls the motor/generator 36 tofunction as a motor to provide torque at the crankshaft 14 to extend theperiod in which fuel is shutoff, thereby increasing fuel economy of thevehicle 10.

While the best modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize various alternative designs and embodiments forpracticing the invention within the scope of the appended claims.

The invention claimed is:
 1. A hybrid powertrain comprising: an engine; a starter motor; a gear train operatively connecting the starter motor with the engine; a motor/generator; a belt drive train operatively connecting the motor/generator with the engine; a first energy storage device operable within a first range of operating voltage; a second energy storage device operable within a second range of operating voltage; wherein the first range of operating voltage is at least partially in common with the second range of operating voltage; a switching device selectively operable in an on-state and an off-state; wherein the first energy storage device and the second energy storage device are operatively connected with one another when the switching device is in the on-state and are operatively disconnected from one another when the switching device is in the off-state; a controller operatively connected to the switching device, the starter motor and the motor/generator; wherein the controller is operable to cause the switching device to be in the off-state and to power both the starter motor and the motor/generator with the first energy storage device during an automatic restart of the engine subsequent to a key start; and a selectively energizable solenoid; wherein the controller is operable to cause the solenoid to be energized to thereby operatively connect the starter motor with the engine through the gear train following cutoff of engine fuel occurring during an automatic shutdown prior to the restart so that the starter motor is operatively connected to the engine prior to the restart.
 2. The hybrid powertrain of claim 1, wherein the controller is operable to cause the motor/generator to be powered as a motor during the entire restart.
 3. The hybrid powertrain of claim 1, wherein the controller is operable to cause the motor/generator to be powered during the restart only after the starter motor has caused the engine to reach to a predetermined speed or after a predetermined amount of time has passed since the starter motor was powered.
 4. The hybrid powertrain of claim 1, wherein the controller is operable to cause the switching device to be in the on-state during the restart when engine operating temperature or operating temperature of one of the energy storage devices is below a predetermined temperature.
 5. The hybrid powertrain of claim 1, wherein the first energy storage device is a lithium-ion battery and the second energy storage device is a lead-acid battery.
 6. The hybrid powertrain of claim 1, wherein the controller is operable to cause the switching device to be in the off-state and to control the motor/generator to function as a generator delivering energy to the first energy storage device to slow an output member of the powertrain in a regenerative braking mode.
 7. The hybrid powertrain of claim 1, wherein the controller is operable to cause the switching device to be in the on-state and to control the motor/generator to function as a generator delivering energy to the second energy storage device when a level of charge of the second energy storage device is below a predetermined level.
 8. The hybrid powertrain of claim 1, wherein the controller is operable to control the motor/generator to function as a motor and cause the switching device to be in the off-state when an output member of the powertrain requires acceleration and engine fuel is cutoff.
 9. The hybrid powertrain of claim 1, further comprising: a vehicle electrical system continuously operatively connected with the second energy storage device regardless of whether the switching device is in the on-state or the off-state and operatively connected with the first energy storage device only when the switching device is in the on-state.
 10. The hybrid powertrain of claim 1, wherein the controller is operable to determine when the restart is complete based on vehicle operating conditions; and wherein the controller is operable to place the switching device in the on-state if the switching device is in the off-state when the restart is complete.
 11. The hybrid powertrain of claim 1, wherein the controller is operable to determine initiation of the key-start of the engine, and the controller is operative to place the switching device in the on-state and the starter motor to be powered by both of the energy storage devices when the key-start is initiated.
 12. A hybrid powertrain comprising: an engine; a starter motor; a gear train operatively connecting the starter motor with the engine; a motor/generator; a belt drive train operatively connecting the motor/generator with the engine; a first energy storage device operable within a first range of operating voltage; a second energy storage device operable within a second range of operating voltage; wherein the first range of operating voltage is at least partially in common with the second range of operating voltage; a switching device selectively operable in an on-state and an off-state; wherein the first energy storage device and the second energy storage device are operatively connected with one another when the switching device is in the on-state and are operatively disconnected from one another when the switching device is in the off-state; a controller operatively connected to the switching device, the starter motor and the motor/generator; wherein the controller is operable to cause the switching device to be in the off-state and to power both the starter motor and the motor/generator with the first energy storage device during an automatic restart of the engine subsequent to a key start; and wherein the controller is operable to determine when the restart is complete based on vehicle operating conditions; and wherein the controller is operable to place the switching device in the on-state if the switching device is in the off-state when the restart is complete. 